1. Field of the Invention
The present invention relates to a planar light source device capable of switching the radiation angle of illuminating light, to a display device provided with this planar light source device and capable of switching the angle range of visibility, to a terminal device equipped with this display device, and to a method for driving the planar light source device.
2. Description of the Related Art
Because of their thin profile, light weight, small size, low energy consumption, and other advantages, display devices that use liquid crystals have been widely deployed and used in a range of devices that includes monitors, televisions (TV: Television), and other large terminal devices; notebook-type personal computers, cash dispensers, vending machines, and other mid-sized terminal devices; and personal TVs, PDAs (Personal Digital Assistance: personal information terminal), mobile telephones, mobile gaming devices, and other small terminal devices. These liquid crystal display devices can be generally classified as transmissive, reflective, or transflective (using transmitted light and reflected light jointly) according to the type of light source used. Energy consumption can be reduced in the reflective type, since it can utilize external light in the display device, but contrast and other aspects of display performance are inferior compared to the transmissive type. Therefore, transmissive and transflective liquid crystal display devices are currently in the mainstream. In transmissive and transflective liquid crystal display devices, a light source is installed on the back surface of a liquid crystal panel, and a display is created using the light emitted by the light source. Specifically, a light source that is separate from the liquid crystal panel is essential in current mainstream liquid crystal display devices.
In the liquid crystal panel that is the primary component of a liquid crystal display device, information is displayed by using an electric field to control the orientation of liquid crystal molecules, and numerous modes have been proposed according to the combination of the type and initial orientation of the liquid crystal molecules, the direction of the electric field, and other characteristics. Among these modes, the modes most often used in a conventional terminal device include an STN (Super Twisted Nematic) mode using a simple matrix structure, and a TN (Twisted Nematic) mode using an active matrix structure. However, a liquid crystal panel that uses these modes has a narrow range of angles in which contrasts can be correctly distinguished, and grayscale inversion occurs outside the optimum viewing position.
This problem of grayscale inversion was relatively insignificant in mobile telephones and other terminal devices when the display content consisted mainly of telephone numbers and other characters. However, with recent technological development, terminal devices have come to display not only text information, but also large amounts of image information. The visibility of images is therefore severely reduced by grayscale inversion. Liquid crystal panels that use a mode having a wide range of angles at which contrast can be correctly distinguished without the occurrence of grayscale inversion are therefore gradually being installed in terminal devices. Liquid crystal panels having this type of mode are referred to generically as wide-viewing-angle liquid crystal panels, and IPS (In-Plane Switching) systems and other horizontal field modes, multi-domain vertical alignment modes, and the like are applied therein. Since gradation can be correctly distinguished in a wide range of angles by using these wide-viewing-angle liquid crystal panels, even though a medium-sized terminal device is basically a personal tool, applications for sharing information with others that can be appreciated by multiple people simultaneously are gradually being developed and installed.
On the other hand, medium-sized terminal devices are characteristically used not only in closed rooms under tight security, but also in public places. It then becomes important to keep displays of private information and confidential information from being viewed by a third party. Particularly in recent years, occasions where private information and confidential information are displayed have increased in conjunction with the development of terminal devices, and demand for eavesdropping prevention techniques is increasing. There is therefore a desire to develop a technique capable of preventing eavesdropping and to enable the display to be viewed only by the user by narrowing the range of angles in which the display is visible; specifically, by narrowing the range of viewing angles.
As described above, a display having a wide range of viewing angles that can be appreciated by multiple people simultaneously, and a display having a narrow range of viewing angles that can be viewed only by the user are both desired. The ability to switch between these two types of displays in a single terminal device is also desired. Therefore, in order to satisfy such requirements, a display device has been proposed in which the light source essential to the liquid crystal display device is designed so that the range of viewing angles can be changed.
FIG. 15 is a schematic sectional view showing the first conventional viewing-angle-controlled liquid crystal display device described in JP-A 5-72529. As shown in FIG. 15, the first conventional viewing-angle-controlled liquid crystal display device 1001 is composed of a liquid crystal element 1170 that is capable of controlling scattering; and a liquid crystal element 1180 that is capable of controlling optical rotation and double refraction properties. The liquid crystal element 1170 that is capable of controlling scattering is composed of substrates 1110 and 1111 that are optically transparent in the visible region, transparent electrodes 1120 and 1121, a scattering liquid crystal 1130, a voltage supply source 1100, and a switch 1190. The liquid crystal element 1180 that is capable of controlling optical rotation and double refraction properties is composed of transparent substrates 1111 and 1112 that are optically transparent in the visible region, transparent electrodes 1122 and 1123, polarizers 1140 and 1141, orientation films 1150 and 1151, a liquid crystal layer 1160 having optical rotation and double refraction properties, a voltage supply source 1101, and a switch 1191. Polymer-dispersed liquid crystal is used as the scattering liquid crystal 1130, and TN liquid crystal is used as the liquid crystal 1180 that is capable of controlling optical rotation and double refraction properties. The polarizers 1140 and 1141 are arranged as a crossed Nicol.
In the first conventional viewing-angle-controlled liquid crystal display device thus configured, a voltage is applied between the transparent electrodes 1122 and 1123, whereby the optical rotation and double refraction properties of the liquid crystal layer 1160 are changed, and this change can be used to control the transmittance of light. In this type of display mode that utilizes optical rotation and double refraction properties, the optical rotation and double refraction properties that essentially affect the incident light differ according to the direction of the viewing angle. A phenomenon therefore occurs in which the luminance and chroma are reduced or inverted depending on the viewing angle. A liquid crystal element 1170 that is capable of controlling scattering is therefore disposed at the top of this type of viewing-angle-dependent liquid crystal element 1180, and the viewing angle dependency is reduced. Specifically, since the liquid crystal molecules are randomly oriented when an electric field is not applied to the liquid crystal 1130 of the liquid crystal element 1170 that is capable of controlling scattering, nearly isotropic scattering occurs throughout the entire range of viewing angles, and a display can be obtained that has little dependency on the viewing angle. When an electric field is applied to the liquid crystal 1130, the liquid crystal molecules orient themselves substantially parallel to the electric field. The light emitted from the liquid crystal element 1180 is therefore emitted without being scattered by the liquid crystal molecules. The visual characteristics do not improve at this time, but when the display need only be correctly recognized by a single user, the viewing angle characteristics resemble those of a conventional TN liquid crystal, and a user can use the display without the display being correctly recognized by another person.
FIG. 16 is a schematic sectional view showing the second conventional viewing-angle-controlled liquid crystal display device described in JP-A 9-244018; and FIG. 17 is a schematic perspective view showing the illumination device used in this viewing-angle-controlled liquid crystal display device. As shown in FIG. 16, the second conventional viewing-angle-controlled liquid crystal display device 2101 is composed of a liquid crystal display element 2102, a scatter control element (scatter control means) 2103, and an illumination device (backlight) 2104. The scatter control element 2103 is disposed between the liquid crystal display element 2102 and the illumination device 2104. As shown in FIG. 17, the illumination device 2104 is provided with an opaque slitted sheet (translucent sheet) 2120 and an irradiating unit 2121. A fluorescent tube or other light source 2122 is provided to the irradiating unit 2121, and a light-emitting surface 2123 for emitting the light from the light source 2122 and guiding the light to the opaque slitted sheet 2120 is formed. A reflecting sheet 2124 for reflecting the light from the light source 2122 is provided in the irradiating unit 2121 on the surface facing the light-emitting surface 2123. In the opaque slitted sheet 2120, a plurality of linear opaque members extending in one direction are arranged parallel to each other on one surface of a translucent sheet. The extension direction of the opaque members coincides with the vertical direction of the display screen.
In the second conventional viewing-angle-controlled liquid crystal display device thus configured, the light emitted from the light source 2122 is emitted from the light-emitting surface 2123 of the irradiating unit 2121, and is radiated to the scatter control element 2103 via the opaque slitted sheet 2120. When the light emitted from the light-emitting surface 2123 passes through the opaque slitted sheet 2120, the opaque slitted sheet 2120 blocks light that is incident from directions that are significantly tilted with respect to the light-incident surface of the opaque slitted sheet 2120. Transmitted light is thereby obtained that is highly parallel to the direction perpendicular to the surface of the opaque slitted sheet 2120. The light emitted from the illumination device 2104 then enters the scatter control element 2103. The scatter control element 2103 controls the scattering properties of the incident light rays according to the presence of an applied voltage. When the scatter control element 2103 is in a scattering state, the light emitted from the illumination device 2104 is scattered by the scatter control element 2103; and when the scatter control element 2103 is in a transparent state, the light from the illumination device 2104 is not scattered.
In the second conventional viewing-angle-controlled liquid crystal display device 2101 configured as described above, the highly collimated light emitted from the illumination device 2104 is scattered by the scatter control element 2103 and caused to enter the liquid crystal display element 2102 when the scatter control element 2103 is in the scattering state. As a result, the light that has passed through the liquid crystal display element 2102 is released in all directions in the viewing angle of the display unit, and it becomes possible to recognize the displayed content also from positions other than the position directly in front of the display unit. In contrast, when the scatter control element 2103 is in the transparent state, the highly collimated light emitted from the illumination device 2104 is caused to enter the liquid crystal display element 2102 while still maintaining a high degree of collimation, without being scattered by the scatter control element 2103. As a result, light is not transmitted to positions where the display unit is viewed at an angle to the left or right in the horizontal direction, the screen is darkened when viewed from such a position, and it becomes impossible to recognize the displayed content. In other words, only an observer who is directly facing the display unit can recognize the displayed content.
As described above, since the scattering properties of the light can be controlled by the scatter control element 2103 in the second conventional viewing-angle-controlled liquid crystal display device 2101 having the abovementioned configuration, the viewing angle characteristics of the displayed content can be controlled. Furthermore, since highly collimated light can be emitted towards the liquid crystal display element 2102 by the illumination device 2104, it is possible to reliably obtain viewing angle characteristics in which only an observer directly facing the display unit can recognize the displayed content when the scatter control element 2103 is placed in the transparent state. Consequently, it is possible to obtain a liquid crystal display device that is capable of arbitrarily switching between a state in which display characteristics are uniformly maintained in all viewing angle directions with little dependence on viewing angle, and a state in which the displayed content can be recognized only from a position directly facing the display unit.
However, in the aforementioned first and second conventional viewing-angle-controlled liquid crystal display devices, there is abnormal flashing that causes discomfort for the user when the switch is made from narrow-angle display having a narrow range of viewing angles to wide-angle display having a wide range of viewing angles.